The invention relates to low noise amplifiers (LNAs). It is particularly related to an LNA having multiple inputs in several frequency bands.
A radio signal sent from a transmitter through a radio channel and received by a receiver suffers from propagation loss. Even though the transmission power from a transmitter such as a base station may be high, the received signal at a receiver such as a mobile station may be very low. According to the GSM standard, a mobile station needs to be able to receive signals of a level of xe2x88x92102 dBm without excessive Bit Error Rate (BER).
The propagation loss is not the only factor having an impact on radio reception. The transmitted signal is further reflected from various surfaces on its way causing multipath propagation. The signals arriving at the receiver via different paths may have opposite or interfering phases and thus may cancel each other out or interfere with each other. Noise from several sources, for example other mobile stations or systems, broadcast systems, radar and non-electromagnetic compatibility (EMC) compliant devices may also be added to the signal.
Low Noise Amplifiers (LNAs) are typically used as a first amplifying stage in radio receivers to amplify a received low-level radio signal to a higher level by adding as little noise as possible to the signal. In communications systems that just use one frequency band one LNA can be optimised for that particular frequency band.
To optimise an LNA for a certain frequency band, inductors are commonly used. Traditionally inductors have been bulky components, whereas a general aim in all electronics design is miniaturisation. Also radio receivers are commonly wanted to be implemented by integrated circuits, where inductance can be realized by planar inductors. Planar inductors, however, still need to use a large area of a semiconductor die compared to transistors.
The diversity of different mobile communications standards around the world has raised a need for mobile stations capable of communicating in several frequency bands. A mobile station can be constructed to work in several communications systems using different frequency bands, such as the 900 MHz GSM (Global System for Mobile Communications) system in Europe and the 1900 MHz TDMA (Time Division Multiple Access) system in US. Several frequency bands can also exist within one single system. For example, GSM systems now work in 900 MHz and 1800 MHz frequency bands in Europe and in 1900 MHz frequency band in the USA. The GSM system will be implemented in the USA also in the 800 MHz frequency band and will, thus, be using two separate frequency bands also in the USA. The third generation cellular communications system known as Universal Mobile Telephone System (UMTS) being specified by Third Generation Partnership Project (3GPP) also uses several frequency bands. A single mobile station working according to two systems, out of which at least one uses several frequency bands, has to use at least three frequency bands.
In order to provide this functionality, it has been proposed to use a dual band receiver such as the dual band receiver 100 shown in FIG. 1. The dual band receiver 100 comprises two receiver branches 103, 104 sharing an antenna 101 for receiving an RF signal and a band switch 102. The receiver branches 103, 104 further comprise band-pass filters 111, 121, LNAs 112, 122 as well as in-phase mixers (I-mixers) 131, 141 and quadrature-phase mixers (Q-mixers) 135, 145 for mixing the RF signal with the respective local oscillator signals. Further, the receiver branches 103, 104 comprise I-filters 133, 143 and Q-filters 137, 147, I-Local Oscillator (I-LO) inputs 132, 142, Q-Local Oscillator (Q-LO) inputs 136, 146 as well as I-outputs 134, 144 and Q-outputs 138, 148. The LNAs 112, 122 may be, for example, variable gain LNAs, as described in the U.S. Pat. Nos. 5,999,056 (Fong) or 6,046,640 (Brunner).
An RF signal sent by a Base Transceiver Station (BTS, not shown) is received by the antenna 101. According to which frequency band the RF signal occupies, it is connected by the band switch 102 to either one of the receiver branches 103, 104. The band switch 102 is controlled by a control signal from a controlling unit (not shown). If a first frequency band is used, the RF signal is connected to the receiver branch 103, where the RF signal is first filtered by the band-pass filter 111. After filtering, the RF signal is amplified by the LNA 112, which has been optimised for the first frequency band. The amplified RF signal is split and routed to the I-mixer 131 and to the Q-mixer 135. An I-LO signal is injected to the I-LO input 132 and a Q-LO signal is injected to the Q-LO input 136. The I-LO signal and the Q-LO signal are at the same frequency, but with a 90-degree phase difference. The I-mixer 131 and the Q-mixer 135 form downconverted signals by mixing the RF-signal and the injected LO-signals. From the I-mixer 131 the downconverted signal is fed to the I-filter 133 and the resultant I-signal is output through the I-output 134. From the Q-mixer 135 the downconverted signal is fed to the Q-filter 137 and the resultant Q-signal is output through Q-output 138.
If the RF signal occupies a second frequency band for which the receiver branch 104 is suitable, the receiver branch 104 will be chosen by the band switch 102. The operation of the receiver branch 104 corresponds to that of the receiver branch 103 as described above.
The solution of the dual band receiver 100 is complicated because it demands two full receiver branches, one for each frequency band which is used. Therefore, alternative dual band receiver arrangements have been proposed.
An alternative dual band receiver 200 known in the prior art and shown in FIG. 2 comprises an antenna 201, a first band switch 202, a band-pass filter 203, 204 for each one of the used frequency bands, a second band switch 206, an LNA 205 and a micro controller 208. The dual band receiver 200 further comprises an I-mixer 211, an I-LO input 212, an I-filter 213, and an I-output 214, as well as a Q-mixer 215, a Q-LO input 216, a Q-filter 217, and a Q-output 218.
An RF signal sent by a BTS (not shown) is received by the antenna 201. According to which frequency band the RF signal occupies it is connected by the first band switch 202 to either one of the band-pass filters 203, 204. The first band switch 202 and the second band switch 206 are controlled synchronously by a control signal from the micro controller 208. If the RF-signal occupies a first frequency band, the RF signal is connected to the band-pass filter 203. After the RF signal has been filtered, the second band switch 206 connects the used band-pass pass filter 203 to the LNA 205 so that the RF signal can be amplified. The amplified RF signal is split and routed to the I-mixer 211 and to the Q-mixer 215. An I-LO signal is injected to the I-LO input 212 and a Q-LO signal is injected to the Q-LO input 216. The I-LO signal and the Q-LO signal are at the same frequency, but with a 90-degree phase difference. The I-mixer 211 and the Q-mixer 215 form downconverted signals each by mixing the RF signal and the injected LO-signals. From the I-mixer 211 the downconverted signal is fed to the I-filter 213 and the resultant I-signal is output through the I-output 214. From the Q-mixer 215 the downconverted signal is fed to the Q-filter 217 and the resultant Q-signal is output through Q-output 218.
If the RF-signal occupies a second frequency band, the RF signal is connected to the band-pass filter 204. After the RF signal has been filtered, the second band switch 206 connects the used band-pass filter 204 to the LNA 205 so that the RF signal can be amplified. The amplified RF signal is handled in a way corresponding to that described in the preceding paragraph.
The dual band receiver 200 has a number of disadvantages. First of all, the additional second band switch 206 introduces extra noise before the LNA 205 on the signal path. Any additional noise before amplification directly decreases signal to noise ratio and should therefore be avoided. This embodiment also makes the optimisation of the LNA 205 more difficult. On the other hand, differential input and output signals are preferred because of the common mode interference rejection. The second band switch 206 makes it difficult to implement differential signals. Thus, the solution of the dual band receiver 200 is not ideal.
The construction of a typical LNA 300 known in the prior art is described with reference to FIG. 3. The LNA 300 comprises a transistor 304 with a load impedance 303 and a degeneration impedance 305. The LNA 300 is operated by power supply 302 and biased by a current fed to the circuit from port Bias 301. A signal to be amplified is fed to RF-in 311 and an amplified signal is output at RF-out 312. The load impedance 303 has an impact on the gain of the LNA 300 and in part also matches the output of the LNA 300 to an external load (not shown). The degeneration impedance 305 is used to linearize the gain of the LNA 300.
If an embodiment of a radio receiver according to FIG. 1 is used, each one of the LNAs 112, 122 need separate load and degeneration circuits as shown by the load impedance 303 and the degeneration impedance 305 in FIG. 3. The load impedance 303 and the degeneration impedance 305 can be implemented by a Inductor-Resistor-Capacitor (LRC) circuitry comprising inductors, resistors and/or capacitors (not shown). In its simplest form the LRC circuitry is often just an inductor, which needs to use a large area of a semiconductor die. The transistor 304 does not need to use a large area in the semiconductor die. The fact of having separate load and degeneration impedance for each LNA leads to a multiplication in the number of impedances 303, 305 and thus to a great increase in the usage of area of the semiconductor die. This problem increases with the number of frequency bands that a radio receiver is constructed to handle.
There is, thus, clearly a need for a multi-band radio receiver LNA architecture that suits well for implementation on an integrated circuit with as small semiconductor die area usage as possible.
In a first aspect of the invention there is provided a low noise amplifier comprising a plurality of amplifying elements, each one of the plurality of amplifying elements being connected to a load impedance and to a degeneration impedance and being configured to amplify signals in a frequency band, the low noise amplifier being characterised in that the plurality of amplifying elements are connected in parallel. the other amplifying elements. In a fifth embodiment of the invention the degeneration impedance is common to the amplifying elements, but at least one of the amplifying elements has a load impedance that is not connected to all of the other amplifying elements.